This invention relates to heat activatable thermoplastic adhesive compositions.
Since most heat activatable thermoplastic adhesives resolidify rapidly upon cooling, the open time (the period after activation of the adhesive during which it remains aggressively tacky and bondable below the activation temperature) is short and bonding operations must be carried out quickly. However, it is often desirable in certain situations to be able to make a bond after the adhesive has cooled, such as where the substrate to be bonded is difficult to align or maneuver into the desired position or where the substrate surfaces are large in area. In these situations, heat activable adhesive compositions which exhibit sustained or delayed tack and which do not develop full adhesive strength immediately upon cooling are particularly suitable.
Various methods of achieving delayed tack are known in the art, including the employment of thermoplastic polymers containing slowly crystallizing segments as described in U.S. Pat. No. 2,653,880 (Hendricks et al.), U.S. Pat. No. 2,653,881 (Vetter) and U.S. Pat. No. 4,059,715 (Pletcher).
In many bonding operations, such as the application of thermal insulation, (e.g., fiberglass) it is necessary to employ adhesive compositions which result in adhesive bonds exhibiting resistance to elevated temperatures. It is known that improved temperature resistance of adhesive compositions can be achieved by the incorporation of crosslinking in the compositions. Particular crosslinking agents known in the art include the polyepoxides and optional epoxy polymerization catalyst described in U.S. Pat. No. 3,723,568 (Hoeschele). Crosslinking in this example is achieved by reactions with available sites in the base polymers.
U.S. Pat. No. 4,137,364 (Ball et al.) teaches crosslinking of an ethylene/vinyl acetate/vinyl alcohol terpolymer using isophthaloyl biscaprolactam or vinyl triethoxy silane. Crosslinking here is achieved before heat reactivation, but a higher level of performance is attained by additional crosslinking induced by a heat cure after application of the adhesive. Another example of thermal crosslinking is the polyamino bis maleimide class of flexible polyimides described in U.S. Pat. No. 4,116,937 (Jones et al.). These compositions can be hot melt extruded up to 300.degree. F. and undergo crosslinking at temperatures above 300.degree. F. In both of these examples of thermal crosslinking, crosslinking is again achieved by reactions of the particular crosslinking agent with available sites of the base polymers.
Crosslinking of the type described above is expected to decrease the tendency of an adhesive composition to exhibit delayed tack. Furthermore, the improvement in adhesive performance (i.e., temperature resistance) brought about by the above type of crosslinking is usually limited by the residual functionality of the base (adhesive) polymer available for crosslinking.